This invention relates to solid electrolyte valve-metal capacitors, and more particularly to tantalum capacitors having a solid manganese dioxide electrolyte.
It is well known that a solid electrolytic valve-metal capacitor may be manufactured by the following method. A sintered porous tantalum body is anodized forming a film of tantalum oxide over all its exposed surfaces. Several coatings of a manganous nitrate solution are applied over the tantalum oxide including regions in the pores of the tantalum body. The body is fired at a temperature of about 400.degree.C, pyrolyzing and transforming the manganous nitrate to a semiconductor, namely manganese dioxide (MnO.sub.2). A layer of graphite is applied over the MnO.sub.2 and a layer of a paste containing silver particles is further applied over the graphite and fired to form a solderable counterelectrode to which a cathode lead wire may be readily attached.
The aforementioned conventional process provides a method for making a conductive and physically secure connection between a substantial portion of the outer coating of MnO.sub.2 and a cathode lead. The steps of applying graphite, silver and then solder solves the major problems inherent in making effective connection to MnO.sub.2.
The use of graphite to contact MnO.sub.2 is universally and exclusively used, to our knowledge, in the manufacture of conventional solid electrolyte MnO.sub.2 valve metal capacitors as represented in the above example. MnO.sub.2 is easily reduced to a non-conducting material. More particularly most metals will reduce MnO.sub.2 as will the vapors of solder fluxes and the polymeric compounds typically used for housings, especially at elevated temperatures. Most capacitors are soldered into the circuits of their application and are exposed, sometimes for several minutes, to temperatures as high as 360.degree.C, which tends to generate vapors of organic materials that are incorporated in the package. Such vapors generally diffuse through the intervening counterelectrode layers. The resulting reduction of the MnO.sub.2 will cause a large increase in the dissipation factor and in the high-frequency impedance of the capacitor. The conventional steps for forming an effective counterelectrode tend to be costly in material and effort, and such conventional solid valve metal capacitors are subject to degradation in performance, especially at elevated temperatures required for soldering the capacitor in a circuit.
A known method for simplifying the counterelectrode structure consists in applying graphite over the MnO.sub.2 as was previously described but then applying a metal such as solder containing large amounts of lead, copper, and brass. A metal wire is embedded in the metal coating, serving as the cathode lead. This structure has the potential for preventing organic vapors from reaching the MnO.sub.2 since a thick layer of solder may be relatively impervious to gasses depending upon the method used for the metallization. However, in any case, the solder is not sealed against the anode lead which would result in shorting the capacitor, and the solder graphite interface is necessarily exposed in the region of the anode. Since it is difficult to produce a tightly adhering thick solder coating to a graphite layer, and further since a graphite layer is permeable to gasses, any organic vapors generated in the package will enter in this anode region.
It is therefore an object of the present invention to provide an improved counterelectrode means in a solid valve metal capacitor.
It is a further object of the present invention to provide a low cost solid valve-metal capacitor.
It is a further object of this invention to provide a solid valve-metal capacitor having superior performance characteristics especially after exposure to elevated temperatures.
It is yet a further object of this invention to provide a solid valve-metal capacitor having a housing of an inexpensive organic material, that is capable of exposure to soldering processes involving temperatures as high as 360.degree.C for several minutes without degradation.